Since a prior art digital filter is adapted to operate with a predetermined dock frequency for processing a certain number of sampled discrete signals separated with an isochronal interval, its characteristics in a relevant frequency region was specified with a constant frequency interval.
For this reason, for filters, whose characteristics were specified with a logarithmic interval in a wide frequency region, were known only those constituted by combinations of a plurality of analogue filters.
Since in the formers, i.e. digital filters, their characteristics are specified with a constant frequency interval, as described above, their resolving power is too rough in the low frequency region and to the contrary unnecessarily fine in the high frequency region. Therefore it is very difficult to make the frequency band of this kind of filters wider.
In general, evaluation or specification of the characteristics of audio devices is effected with logarithmic intervals in the frequency region. Consequently, in the case whose digital filters, e.g. FIR (finite impulse response) type filters were used, for the reason described above it was practically not possible to specify the characteristics with a high precision for the low frequency region and to the contrary the characteristics were specified with an unnecessarily high precision for the high frequency region. Therefore, design to realize circuits of the digital filter described above was very complicated and not efficient. In particular, in the case where it was tried to realize a circuit having complex frequency characteristics such as a graphic equalizer by means of prior art FIR type filters, such a restriction cannot be avoided and in fact any graphic equalizers utilizing FIR type filters have never been used in practice.
On the other hand, since the latter, i.e. analogue filters, were so constructed that they were driven according to a method, by which analogue active filters were added to or multiplied by each other, interference was produced between different filters or it was difficult to specify the phase and the amplitude thereof independently. In particular, it was very difficult to specify arbitrarily their phase characteristics, thereof.